Computer system components, such as central processing units (CPUs), chipsets, graphics cards, and hard drives, produce large amounts of heat during operation. This heat must be dissipated in order to keep these components within safe operating temperatures. Overheated components generally exhibit a shorter life span and may also cause malfunction of the computer system.
The risk of overheating increases with increasing density of computer system components. In a typical blade server, a large number of heat generating blades may be closely placed in a single system enclosure. Limited open space in the system enclosure results in reduced air circulation and correspondingly reduced heat dissipation.
Because of these heat loads, many blade server enclosures include a thermal management system that uses both active (i.e., convection) and passive (e.g., heat sinks) cooling. Convection cooling generally relies on one or more fans that operate at either fixed or variable speeds. A variable speed fan generally is best for matching air flow to heat load. However, the setting of this variable fan speed presents a design problem. Ideally, the cooling fans would operate at a speed that does not waste energy while maintaining the blades at the optimum operating temperature. More specifically, the blades may be cooled simply by operating the fans at a constant high speed. This approach causes a waste of energy when the blades are not operating at their maximum capacity. One approach is to use temperature-sensing devices in the fans, where the temperature-sensing devices directly measure how much heat the server generates in the exhaust air stream. When the fan detects that the server exhaust temperatures are increasing, the fan's microcontroller can increase fan speed. However, this approach has its limitations because servers can heat up very quickly, and the server's ROM could trip on a thermal shutdown before the fans could create enough additional cooling.